How to get outer part of the mesh?
Are they two separate chunks of mesh? It looks like they are from the images.

If so select only one vertex from the outer shell, press Ctrl+L to Select All Linked and it should select all the outer part.

Alternatively in Edit Mode select all faces, press P > Loose parts and it should break your object apart by all separate chunks.

If they are not separate chunks then I'm afraid you don't really have a chioce other than manually selecting everything.

Under Edit Mode you can enable the option Limit selection to Visible in the 3DView header button, and use regular selection methods to select only the visible parts of the mesh. With some luck the 'inner' parts are occluded and wont be selected.